All-solid-state battery

ABSTRACT

Provided is an all-solid-state battery with high charge-discharge efficiency. Disclosed is an all-solid-state battery, wherein the all-solid-state battery comprises a cathode comprising a cathode layer, an anode comprising an anode layer, and a solid electrolyte layer disposed between the cathode layer and the anode layer; wherein the anode layer contains at least one selected from the group consisting of a lithium metal and a lithium alloy; and wherein a protective layer comprising a composite metal oxide represented by Li-M-O (where M is at least one metal element selected from the group consisting of Mg, Au, Al and Sn) is disposed between the anode layer and the solid electrolyte layer.

TECHNICAL FIELD

The disclosure relates to an all-solid-state battery.

BACKGROUND

In recent years, with the rapid spread of IT and communication devicessuch as personal computers, camcorders and cellular phones, greatimportance has been attached to the development of batteries that isusable as the power source of such devices. In the automobile industry,etc., high-power and high-capacity batteries for electric vehicles andhybrid vehicles are under development.

Of various kinds of batteries, a lithium secondary battery has attractedattention for the following reasons: since it uses lithium, which is ametal having the largest ionization tendency, as the anode, thepotential difference between the cathode and the anode is large, andhigh output voltage is obtained.

Also, an all-solid-state battery has attracted attention, since it usesa solid electrolyte as the electrolyte present between the cathode andthe anode, in place of a liquid electrolyte containing an organicsolvent.

Patent Literature 1 discloses a battery in which a layer containing oneor more elements selected from the group consisting of Cr, Ti, W, C, Ta,Au, Pt, Mn and Mo is arranged between a collector foil and an electrodebody.

Patent Literature 2 discloses a solid battery in which a metal oxidelayer containing an oxide of at least one metal element selected fromthe group consisting of Cr, In, Sn, Zn, Sc, Ti, V, Mn, Fe, Co, Ni, Cuand W, is formed at least on an interface between a current collectorand a cathode and/or anode adjacent to the current collector.

Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No.2012-049023

Patent Literature 2: JP-A No. 2009-181901

An all-solid-state battery in which the anode contains a lithium metal,has the following problem: even if the all-solid-state battery has aconventionally-known battery structure, the charge-discharge efficiencyof the all-solid-state battery is low.

SUMMARY

In light of the above circumstances, an object of the disclosedembodiments is to provide an all-solid-state battery with highcharge-discharge efficiency.

In a first embodiment, there is provided an all-solid-state battery,

wherein the all-solid-state battery comprises a cathode comprising acathode layer, an anode comprising an anode layer, and a solidelectrolyte layer disposed between the cathode layer and the anodelayer;

wherein the anode layer contains at least one selected from the groupconsisting of a lithium metal and a lithium alloy; and

wherein a protective layer comprising a composite metal oxiderepresented by Li-M-O (where M is at least one metal element selectedfrom the group consisting of Mg, Au, Al and Sn) is disposed between theanode layer and the solid electrolyte layer.

The thickness of the protective layer may be from 30 nm to 300 nm.

The composite metal oxide may be represented by Li—Mg—O.

According to the disclosed embodiments, an all-solid-state battery withhigh charge-discharge efficiency is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 is a schematic sectional view of an example of theall-solid-state battery of the disclosed embodiments when the battery isfully charged;

FIG. 2 is a graph showing the average charge-discharge efficiencies ofevaluation batteries 1 to 5;

FIG. 3 is a graph showing the resistance increase rates of theevaluation batteries 1 to 5; and

FIG. 4 is a view showing the results of XPS analysis in which elementalanalysis was carried out to 500 nm in the depth direction from the Cufoul-side surface of the solid electrolyte layer contained in theevaluation battery 4 after the first charge-discharge cycle of Example4.

DETAILED DESCRIPTION

The all-solid-state battery of the disclosed embodiments is anall-solid-state battery,

wherein the all-solid-state battery comprises a cathode comprising acathode layer, an anode comprising an anode layer, and a solidelectrolyte layer disposed between the cathode layer and the anodelayer;

wherein the anode layer contains at least one selected from the groupconsisting of a lithium metal and a lithium alloy; and

wherein a protective layer comprising a composite metal oxiderepresented by Li-M-O (where M is at least one metal element selectedfrom the group consisting of Mg, Au, Al and Sn) is disposed between theanode layer and the solid electrolyte layer.

In the disclosed embodiments, “lithium secondary battery” means abattery in which at least one of a lithium metal and a lithium alloy isused as the anode active material and a lithium metalprecipitation-dissolution reaction is used as an anode reaction.

In the disclosed embodiments, “when the all-solid-state battery is fullycharged” means that the SOC (state of charge) value of theall-solid-state battery is 100%. The SOC means the percentage of thecharge capacity with respect to the full charge capacity of the battery.The full charge capacity is a SOC of 100%.

For example, the SOC may be estimated from the open circuit voltage(OCV) of the all-solid-state battery.

The techniques disclosed in Patent Literatures 1 and 2 focused on theinterface between the current collector and the anode layer. However,since the reaction in the interface between the solid electrolyte layerand the anode layer is faster than the reaction in the interface betweenthe current collector and the anode layer, the resistance increase rateof the all-solid-state battery in which no treatment is given to theinterface between the solid electrolyte layer and the anode layer, isthought to increase along with repeated charge-discharge cycles.

A lithium metal is highly reductive and is problematic in that whencontained in the anode layer, it reacts with the solid electrolytecontained in the solid electrolyte layer being in contact with the anodelayer and results in the formation of a resistive layer between theanode layer and the solid electrolyte layer.

In addition, there is the following problem: during the all-solid-statebattery is charged/discharged, the resistance of the resistive layer isincreased, thereby decreasing the battery characteristics of theall-solid-state battery, such as charge-discharge efficiency anddurability.

The disclosed embodiments provide an all-solid-state battery which is,by disposing the protective layer comprising the composite metal oxidein the interface between the anode layer and the solid electrolytelayer, configured to suppress an increase in the resistance of theinterface between the anode layer and the solid electrolyte layer, andwhich is provided with excellent battery characteristics.

As shown in FIG. 1, an all-solid-state battery 100 comprises a cathode16 comprising a cathode layer 12 and a cathode current collector 14, ananode 17 comprising an anode layer 13 and an anode current collector 15,a solid electrolyte layer 11 disposed between the cathode layer 12 andthe anode layer 13, and a protective layer 18 disposed between the solidelectrolyte layer 11 and the anode layer 13. When the anode layer 13 iscomposed of a lithium metal, the anode layer 13 may be dissolved andlost in the all-solid-state battery 100 before being initially chargedor after being fully discharged.

Protective Layer

The protective layer is disposed between the anode layer and the solidelectrolyte layer.

The protective layer comprises a composite metal oxide represented byLi-M-O (where M is at least one metal element selected from the groupconsisting of Mg, Au, Al and Sn).

The composite metal oxide may be an oxide of an alloy of Li and at leastone metal element selected from the group consisting of Mg, Au, Al andSn. As the oxide, examples include, but are not limited to, an oxide ofa Li—Mg alloy (Li—Mg—O), an oxide of a Li-gold alloy (Li—Au—O), an oxideof a Li—Al alloy (Li—Al—O), and an oxide of a Li—Sn alloy (Li—Sn—O).From the viewpoint of increasing the charge-discharge efficiency of theall-solid-state battery, the oxide may be an oxide of a Li—Mg alloy(Li—Mg—O).

The composite metal oxide may be in any one of a solid solution state,an eutectic state and an intermetallic compound state.

The Li-M-O may be a composite metal oxide represented by Li_(x)-M-O_(y)(where M is at least one metal element selected from the groupconsisting of Mg, Au, Al and Sn; 0<x≤4; and 1≤y≤2).

A Li—Mg—O example is Li₂MgO, which is presumed to be produced by, forexample, charging the all-solid-state battery and the resulting reactionrepresented by the following formula (1):MgO+2Li→Li₂MgO  Formula (1):

A Li—Au—O example is Li₂AuO, which is presumed to be produced by, forexample, charging the all-solid-state battery and the resulting reactionrepresented by the following formula (2):AuO+2Li→Li₂AuO  Formula (2):

A Li—Al—O example is 2Li₃AlO_(1.5), which is presumed to be produced by,for example, charging the all-solid-state battery and the resultingreaction represented by the following formula (3):Al₂O₃+6Li→2Li₃AlO_(1.5)  Formula (3):

A Li—Sn—O example is Li₄SnO₂, which is presumed to be produced by, forexample, charging the all-solid-state battery and the resulting reactionrepresented the following formula (4):SnO₂+4Li→Li₄SnO₂  Formula (4):

The percentage of the elements contained in the composite metal oxidemay vary depending on the type of the contained metal M, the degree ofoxidation, etc.

For the percentage of the Li element in the composite metal oxide, forexample, the lower limit may be 30.0 atomic % or more, and the upperlimit may be 99.9 atomic % or less. The percentage of any element in thecomposite metal oxide may be calculated by elemental analysis of thecomposite metal oxide by X-ray photoelectron spectroscopy (XPS). Thepercentage of the elements in the composite metal oxide may becalculated by elemental analysis of the composite metal oxide by XPSwhile the composite metal oxide is in an absolutely dissolved state.

From the viewpoint of increasing the charge-discharge efficiency of theall-solid-state battery, the thickness of the protective layer may befrom 30 nm to 300 nm.

The method for forming the protective layer is not particularly limited.For example, the protective layer comprising the composite metal oxidemay be formed by vacuum deposition of the composite metal oxide on onesurface of the solid electrolyte layer or anode layer, using an electronbeam evaporation device.

Another method for forming the protective layer may be the followingmethod, for example.

First, using an electron beam evaporation device, a metal layercontaining at least one metal selected from the group consisting of Mg,Au, Al and Sn, is formed by vacuum deposition of the metal on onesurface of the solid electrolyte layer or anode current collector. Then,a cathode layer containing at least one kind of cathode active materialselected from the group consisting of a lithium metal, a lithium alloyand a lithium compound, is prepared. The cathode layer, the solidelectrolyte layer, the metal layer and the anode current collector aredisposed in this order to prepare a battery precursor. By charging thebattery precursor, lithium ions are transferred from the cathode layerto the metal layer and reacted with the metal in the metal layer. Bythis reaction, the protective layer comprising the Li-M-O compositemetal oxide is formed on the metal layer-side surface of the solidelectrolyte layer. Accordingly, the protective layer is obtained.

In general, the surface of the metal layer is covered with the oxidelayer. Accordingly, by charging the battery precursor, the oxide layeron the surface of the metal layer reacts with lithium ions to form, onthe solid electrolyte layer surface, a composite metal oxide layercontaining a Li-M-O alloy. The composite metal oxide layer is morestable than the lithium metal, functions as the protective layer forsuppressing a reaction between the lithium metal and the solidelectrolyte, and provides high lithium ion conductivity due to itslithium element content. Accordingly, once the protective layer isformed, the protective layer is not lost even if the precursor batteryis discharged. In the charging of the precursor battery, once theprotective layer is formed, using the protective layer as aprecipitation starting point, at least one of the lithium metal and thelithium alloy is further precipitated on the protective layer to formthe anode layer. Accordingly, the all-solid-state battery in which theprotective layer is disposed between the anode layer and the solidelectrolyte layer, is obtained.

Cathode

The cathode comprises the cathode layer. As needed, it comprises acathode current collector.

The cathode layer contains the cathode active material. As optionalcomponents, the cathode layer may contain a solid electrolyte, anelectroconductive material and a binder, for example.

The type of the cathode active material is not particularly limited. Thecathode active material can be any type of material that is usable as anactive material for all-solid-state batteries. When the all-solid-statebattery is an all-solid-state lithium secondary battery, as the cathodeactive material, examples include, but are not limited to, a lithiummetal (Li), a lithium alloy, LiCoO₂, LiNi_(x)Co_(1−x)O₂ (0<x<1),LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂, LiMnO₂, different element-substitutedLi—Mn spinels (such as LiMn_(1.5)Ni_(0.5)O₄, LiMn_(1.5)Al_(0.5)O₄,LiMn_(1.5)Mg_(0.5)O₄, LiMn_(1.5)Co_(0.5)O₄, LiMn_(1.5)Fe_(0.5)O₄ andLiMn_(1.5)Zn_(0.5)O₄), lithium titanates (such as Li₄Ti₅O₁₂), lithiummetal phosphates (such as LiFePO₄, LiMnPO₄, LiCoPO₄ and LiNiPO₄),lithium compounds (such as LiCoN, Li₂SiO₃ and Li₄SiO₄), transition metaloxides (such as V₂O₅ and MoO₃), TiS₂, Si, SiO₂ and lithium storageintermetallic compounds (such as Mg₂Sn, Mg₂Ge, Mg₂Sb and Cu₃Sb). As thelithium alloy, examples include, but are not limited to, Li—Au, Li—Mg,Li—Sn, Li—Si, Li—Al, Li—Ge, Li—Sb, Li—B, Li—C, Li—Ca, Li—Ga, Li—As,Li—Se, Li—Ru, Li—Rh, Li—Pd, Li—Ag, Li—Cd, Li—Ir, Li—Pt, Li—Hg, Li—Pb,Li—Bi, Li—Zn, Li—Tl, Li—Te, Li—At and Li—In.

The form of the cathode active material is not particularly limited. Itmay be a particulate form.

A coating layer containing a Li ion conducting oxide, may be formed onthe surface of the cathode active material. This is because a reactionbetween the cathode active material and the solid electrolyte can besuppressed.

As the Li ion conducting oxide, examples include, but are not limitedto, LiNbO₃, Li₄Ti₅O₁₂ and Li₃PO₄. The thickness of the coating layer is0.1 nm or more, for example, and it may be 1 nm or more. On the otherhand, the thickness of the coating layer is 100 nm or less, for example,and it may be 20 nm or less. Also, for example, 70% or more or 90% ormore of the cathode active material surface may be coated with thecoating layer.

The content of the solid electrolyte in the cathode layer is notparticularly limited. When the total mass of the cathode layer isdetermined as 100 mass %, the content of the solid electrolyte may be ina range of from 1 mass % to 80 mass %, for example.

As the solid electrolyte, examples include, but are not limited to, anoxide-based solid electrolyte and a sulfide-based solid electrolyte.

As the sulfide-based solid electrolyte, examples include, but are notlimited to, Li₂S—P₂S₅, Li₂S—SiS₂, LiX—Li₂S—SiS₂, LiX—Li₂S—P₂S₅,LiX—Li₂O—Li₂S—P₂S₅, LiX—Li₂S—P₂O₅, LiX—Li₃PO₄—P₂S₅ and Li₃PS₄. The“Li₂S—P₂S₅” means a material composed of a raw material compositioncontaining Li₂S and P₂S₅, and the same applies to other solidelectrolytes. Also, “X” in the “LiX” means a halogen element. The LiXcontained in the raw material composition may be one or more kinds. Whentwo or more kinds of LiX are contained in the raw material composition,the mixing ratio is not particularly limited.

The molar ratio of the elements in the sulfide-based solid electrolytecan be controlled by controlling the contents of the elements containedin raw materials. The molar ratio and composition of the elements in thesulfide-based solid electrolyte can be measured by inductively coupledplasma atomic emission spectroscopy, for example.

The sulfide-based solid electrolyte may be sulfide glass, crystallizedsulfide glass (glass ceramics) or a crystalline material obtained bydeveloping a solid state reaction of the raw material composition.

The crystal state of the sulfide-based solid electrolyte can beconfirmed by X-ray powder diffraction measurement using CuKα radiation,for example.

The sulfide glass can be obtained by amorphizing a raw materialcomposition (such as a mixture of Li₂S and P₂S₅). The raw materialcomposition can be amorphized by mechanical milling, for example. Themechanical milling may be dry mechanical milling or wet mechanicalmilling. The mechanical milling may be the latter because attachment ofthe raw material composition to the inner surface of a container, etc.,can be prevented.

The mechanical milling is not particularly limited, as long as it is amethod for mixing the raw material composition by applying mechanicalenergy thereto. The mechanical milling may be carried out by, forexample, a ball mill, a vibrating mill, a turbo mill, mechanofusion, ora disk mill. The mechanical milling may be carried out by a ball mill,or it may be carried out by a planetary ball mill. This is because thedesired sulfide glass can be efficiently obtained.

The glass ceramics can be obtained by heating the sulfide glass, forexample.

For the heating, the heating temperature may be a temperature higherthan the crystallization temperature (Tc) of the sulfide glass, which isa temperature observed by thermal analysis measurement. In general, itis 195° C. or more. On the other hand, the upper limit of the heatingtemperature is not particularly limited.

The crystallization temperature (Tc) of the sulfide glass can bemeasured by differential thermal analysis (DTA).

The heating time is not particularly limited, as long as the desiredcrystallinity of the glass ceramics is obtained. For example, it is in arange of from one minute to 24 hours, or it may be in a range of fromone minute to 10 hours.

The heating method is not particularly limited. For example, a firingfurnace may be used.

As the oxide-based solid electrolyte, examples include, but are notlimited to, Li_(6.25)La₃Zr₂Al_(0.25)O₁₂, Li₃PO₄, andLi_(3+x)PO_(4−x)N_(x) (1≤x≤3).

From the viewpoint of handling, the form of the solid electrolyte may bea particulate form.

The average particle diameter (D₅₀) of the solid electrolyte particlesis not particularly limited. The lower limit may be 0.5 μm or more, andthe upper limit may be 2 μm or less.

As the solid electrolyte, one or more kinds of solid electrolytes may beused. In the case of using two or more kinds of solid electrolytes, theymay be mixed together.

In the disclosed embodiments, unless otherwise noted, the averageparticle diameter of particles is a volume-based median diameter (D₅₀)measured by laser diffraction/scattering particle size distributionmeasurement. Also in the disclosed embodiments, the median diameter(D₅₀) of particles is a diameter at which, when particles are arrangedin ascending order of their particle diameter, the accumulated volume ofthe particles is half (50%) the total volume of the particles (volumeaverage diameter).

As the electroconductive material, a known electroconductive materialmay be used. As the electroconductive material, examples include, butare not limited to, a carbonaceous material and metal particles. Thecarbonaceous material may be at least one selected from the groupconsisting of carbon nanotube, carbon nanofiber and carbon blacks suchas acetylene black (AB) and furnace black. Of them, from the viewpointof electron conductivity, the electroconductive material may be at leastone selected from the group consisting of carbon nanotube and carbonnanofiber. The carbon nanotube and carbon nanofiber may be vapor-growncarbon fiber (VGCF). As the metal particles, examples include, but arenot limited to, particles of Ni, particles of Cu, particles of Fe andparticles of SUS.

The content of the electroconductive material in the cathode layer isnot particularly limited.

As the binder, examples include, but are not limited to,acrylonitrile-butadiene rubber (ABR), butadiene rubber (BR),polyvinylidene fluoride (PVdF) and styrene-butadiene rubber (SBR). Thecontent of the binder in the cathode layer is not particularly limited.

The thickness of the cathode layer is not particularly limited.

The cathode layer can be formed by a conventionally-known method.

For example, a cathode layer slurry is produced by putting the cathodeactive material and, as needed, other components in a solvent and mixingthem. The cathode layer slurry is applied on one surface of a supportsuch as the cathode current collector. The applied slurry is dried,thereby forming the cathode layer.

As the solvent, examples include, but are not limited to, butyl acetate,butyl butyrate, heptane and N-methyl-2-pyrrolidone.

The method for applying the cathode layer slurry on one surface of thesupport such as the cathode current collector, is not particularlylimited. As the method, examples include, but are not limited to, adoctor blade method, a metal mask printing method, an electrostaticcoating method, a dip coating method, a spray coating method, a rollercoating method, a gravure coating method and a screen printing method.

The support may be appropriately selected from self-supporting supports,and it is not particularly limited. For example, a metal foil such as Cuand Al may be used as the support.

The cathode layer may be formed by another method such aspressure-forming a powdered cathode mix that contains the cathode activematerial and, as needed, other components. In the case ofpressure-forming the powdered cathode mix, generally, a press pressureof about 1 MPa or more and about 600 MPa or less is applied.

The pressure applying method is not particularly limited. As the method,examples include, but are not limited to, pressing by use of a platepress machine, a roll press machine or the like.

As the cathode current collector, a conventionally-known metal that isusable as a current collector in all-solid-state batteries, may be used.As the metal, examples include, but are not limited to, a metal materialcontaining one or more elements selected from the group consisting ofCu, Ni, Al, V, Au, Pt, Mg, Fe, Ti, Co, Cr, Zn, Ge and In.

The form of the cathode current collector is not particularly limited.As the form, examples include, but are not limited to, various kinds offorms such as a foil form and a mesh form.

The form of the whole cathode is not particularly limited. It may be asheet form. In this case, the thickness of the whole cathode is notparticularly limited and may be determined depending on desiredperformance.

Solid Electrolyte Layer

The solid electrolyte layer contains at least a solid electrolyte.

As the solid electrolyte contained in the solid electrolyte layer, aconventionally-known solid electrolyte that is usable in all-solid-statebatteries, can be appropriately used. As such a solid electrolyte,examples include, but are not limited to, a solid electrolyte that canbe incorporated in the above-described cathode layer.

As the solid electrolyte, one or more kinds of solid electrolytes may beused. In the case of using two or more kinds of solid electrolytes, theymay be mixed together, or they may be formed into layers to obtain amulti-layered structure.

The proportion of the solid electrolyte in the solid electrolyte layeris not particularly limited. For example, it may be 50 mass % or more,may be in a range of 60 mass % or more and 100 mass % or less, may be ina range of 70 mass % or more and 100 mass % or less, or may be 100 mass%.

From the viewpoint of exerting plasticity, etc., a binder can beincorporated in the solid electrolyte layer. As the binder, examplesinclude, but are not limited to, a binder that can be incorporated inthe above-described cathode layer. However, the content of the binder inthe solid electrolyte layer may be 5 mass % or less, from the viewpointof, for example, preventing excessive aggregation of the solidelectrolyte and making it possible to form the solid electrolyte layerin which the solid electrolyte is uniformly dispersed, for the purposeof easily achieving high power output.

The thickness of the solid electrolyte layer is not particularlylimited. It is generally 0.1 μm or more and 1 mm or less.

As the method for forming the solid electrolyte layer, examples include,but are not limited to, pressure-forming a powdered solid electrolytematerial that contains the solid electrolyte and, as needed, othercomponents. In the case of pressure-forming the powdered solidelectrolyte material, generally, a press pressure of about 1 MPa or moreand about 600 MPa or less is applied.

The pressing method is not particularly limited. As the method, examplesinclude, but are not limited to, those exemplified above in theformation of the cathode layer.

Anode

The anode comprises at least an anode layer. As needed, it comprises ananode current collector.

The anode layer contains an anode active material.

As the anode active material, examples include, but are not limited to,a lithium metal (Li) and a lithium alloy. As the lithium alloy, examplesinclude, but are not limited to, Li—Au, Li—Mg, Li—Sn, Li—Al, Li—B, Li—C,Li—Ca, Li—Ga, Li—Ge, Li—As, Li—Se, Li—Ru, Li—Rh, Li—Pd, Li—Ag, Li—Cd,Li—In, Li—Sb, Li—Ir, Li—Pt, Li—Hg, Li—Pb, Li—Bi, Li—Zn, Li—Tl, Li—Te andLi—At.

As long as the lithium metal (Li) or the lithium alloy is contained asan anode active material and as a main component in the anode layer,another conventionally-known anode active material may be contained. Inthe disclosed embodiments, the “main component” means a component thataccounts for 50 mass % or more of the total mass of the anode layer.

As the anode current collector, a known metal that is usable as acurrent collector in all-solid-state batteries, can be used. As such ametal, examples include, but are not limited to, the above-exemplifiedmetal materials that are usable as the cathode current collector.

The thickness of the whole anode is not particularly limited.

As needed, the all-solid-state battery comprises an outer casing forhousing the cathode, the anode and the solid electrolyte layer.

As the form of the outer casing, examples include, but are not limitedto, a laminate form.

The material for the outer casing is not particularly limited, as longas it is a material that is stable in electrolytes. As the material,examples include, but are not limited to, resins such as polypropylene,polyethylene and acrylic resin.

The all-solid-state battery may be an all-solid-state lithium secondarybattery.

As the form of the all-solid-state battery, examples include, but arenot limited to, a coin form, a laminate form, a cylindrical form and asquare form.

The all-solid-state battery production method of the disclosedembodiments may be as follows, for example. First, the solid electrolytelayer is formed by pressure-forming a powdered solid electrolytematerial. Next, the cathode layer is obtained by pressure-forming apowdered cathode mix that contains the cathode active materialcontaining the lithium element on one surface of the solid electrolytelayer. Then, by vacuum deposition of the composite metal oxide using theelectron beam evaporation device, the protective layer comprising thecomposite metal oxide is formed on the opposite surface of the solidelectrolyte layer to the surface on which the cathode layer is formed.In addition, the anode active material is disposed on the protectivelayer to obtain the anode layer. Accordingly, a cathode layer-solidelectrolyte layer-protective layer-anode layer assembly is obtained. Asneeded, a current collector is attached to the assembly, therebyobtaining the battery precursor.

In this case, the press pressure applied for pressure-forming thepowdered solid electrolyte material and the powdered cathode mix, isgenerally about 1 MPa or more and about 600 MPa or less.

The pressing method is not particularly limited. As the pressing method,examples include, but are not limited to, those exemplified above in theformation of the cathode layer.

EXAMPLES Example 1

Using an electron beam evaporation device, Sn was evaporated to athickness of 100 nm on one surface of a Cu foil, thereby forming a metallayer.

As a sulfide-based solid electrolyte, 101.7 mg of a Li₂S—P₂S₅-basedmaterial containing LiBr and LiI was prepared. The sulfide-based solidelectrolyte was pressed at a pressure of 6 ton/cm², thereby obtaining asolid electrolyte layer (thickness 500 μm).

Next, a Li metal foil (thickness 150 μm) was disposed on one surface ofthe solid electrolyte layer. The Cu foil having the metal layer formedon one surface thereof, was disposed on the opposite surface of thesolid electrolyte layer to the surface on which the Li metal foil wasdisposed, to ensure that the solid electrolyte layer and the metal layerwere in contact with each other. They were pressed at a pressure of 1ton/cm², thereby forming an evaluation battery 1 comprising the Li metalfoil, the solid electrolyte layer, the metal layer and the Cu foil inthis order.

Example 2

An evaluation battery 2 was obtained in the same manner as Example 1,except that using the electron beam evaporation device, Al wasevaporated to a thickness of 100 nm on one surface of the Cu foil, inplace of Sn, thereby forming the metal layer.

Example 3

An evaluation battery 3 was obtained in the same manner as Example 1,except that using the electron beam evaporation device, Au wasevaporated to a thickness of 100 nm on one surface of the Cu foil, inplace of Sn, thereby forming the metal layer.

Example 4

An evaluation battery 4 was obtained in the same manner as Example 1,except that using the electron beam evaporation device, Mg wasevaporated to a thickness of 100 nm on one surface of the Cu foil, inplace of Sn, thereby forming the metal layer.

Comparative Example 1

An evaluation battery 5 was obtained in the same manner as Example 1,except that the metal layer was not formed on one surface of the Cufoil.

Charge-Discharge Test

The evaluation battery 1 was left to stand for one hour in a thermostatbath at 25° C. to uniform the temperature of the inside of theevaluation battery 1.

Next, the evaluation battery 1 was charged at a constant current with acurrent density of 435 μA/cm² to form, in the interface between thesolid electrolyte layer and the metal layer, a protective layercontaining a composite metal oxide (Li—Sn—O) obtained by a reaction ofan Sn oxide layer on one surface of the metal layer and lithium ionsthat were formed by the dissolution of the Li metal foil and thentransferred to the metal layer side through the solid electrolyte layer.The evaluation battery 1 was kept charged to precipitate a Li metal onthe protective layer. The charging of the evaluation battery 1 wasterminated when the charge capacity of the evaluation battery 1 reached4.35 mAh/cm². Accordingly, the evaluation battery 1 became anall-solid-state lithium secondary battery comprising the protectivelayer between the solid electrolyte layer and the anode layer containingthe precipitated lithium metal. After 10 minutes passed, the evaluationbattery 1 was discharged at a constant current with a current density of435 μA/cm² to dissolve the Li metal precipitated on the protectivelayer. The discharging of the evaluation battery 1 was terminated whenthe voltage of the evaluation battery 1 reached 1.0 V.

The charge-discharge efficiency of the evaluation battery 1 was obtainedby the following formula.Charge-discharge efficiency (%)=(Discharge capacity/Charge capacity)×100

Then, the time between the start of the charging and the end of thedischarging was determined as one cycle, and a total of 4 cycles ofcharging and discharging were repeated. The average charge-dischargeefficiency of the evaluation battery 1 was calculated from thethus-obtained charge-discharge efficiencies of the evaluation battery 1.The result is shown in Table 1 and FIG. 2.

The average charge-discharge efficiencies of the evaluation batteries 2to 5 were calculated in the same manner as the evaluation battery 1. Theresults are shown in Table 1 and FIG. 2. The protective layer of theevaluation battery 2 contained Li—Al—O as the composite metal oxide inplace of Li—Sn—O. The protective layer of the evaluation battery 3contained Li—Au—O as the composite metal oxide in place of Li—Sn—O. Theprotective layer of the evaluation battery 4 contained Li—Mg—O as thecomposite metal oxide in place of Li—Sn—O.

Resistance Measurement

In the charging of the first cycle of the evaluation battery 1, when thecharge capacity of the evaluation battery 1 reached 1 mAh/cm², thevoltage of the evaluation battery 1 was read as Li precipitationovervoltage, and the resistance value of the first cycle of theevaluation battery was obtained by the following formula. The result isshown in Table 1.Resistance (Ω/cm²)=Li precipitation overvoltage (V)/(435×10⁻⁶ (A/cm²))

In the charging of the 4th cycle of the evaluation battery 1, when thecharge capacity of the evaluation battery 1 reached 1 mAh/cm², thevoltage of the evaluation battery 1 was read as Li precipitationovervoltage, and the resistance value of the 4th cycle of the evaluationbattery 1 was obtained by the above formula. The result is shown inTable 1.

Then, from the resistance values of the first and 4th cycles of theevaluation battery 1, the resistance increase rate along with charge anddischarge of the evaluation battery 1 was calculated by the followingformula. The result is shown in Table 1 and FIG. 3.Resistance increase rate (%)=[(Resistance value of 4th cycle−Resistancevalue of 1st cycle)/Resistance value of 1st cycle]×100

For the evaluation batteries 2 to 5, the resistance value of the firstcycle, the resistance value of the 4th cycle, Δ (obtained by subtractingthe resistance value of the first cycle from the resistance value of the4th cycle) and the resistance increase rate were calculated in the samemanner as the evaluation battery 1. The results are shown in Table 1 andFIG. 3.

XPS Measurement

After the first charge-discharge cycle of the evaluation battery 4 ofExample 4, elemental analysis of the Cu foil-side surface of the solidelectrolyte layer of the evaluation battery 4, was carried out in thedepth direction from the surface by X-ray photoelectron spectroscopy(XPS). The scan rate was set to 25 nm/min, and the scan depth was set to500 nm. That is, a thickness (500 nm) that was one-tenth of thethickness of the solid electrolyte layer (500 μm) from the Cu foil-sidesurface of the solid electrolyte layer, was scanned.

The result is shown in FIG. 4. FIG. 4 is a view showing the results ofXPS analysis in which elemental analysis was carried out to 500 nm inthe depth direction from the Cu foil-side surface of the solidelectrolyte layer contained in the evaluation battery 4 after the firstcharge-discharge cycle of Example 4.

As shown in FIG. 4, the presence of Li, Mg and oxygen elements wasconfirmed from the Cu foil-side surface of the solid electrolyte layerto a depth of about 300 nm thereof.

Accordingly, the formation of the protective layer having a thickness of300 nm and containing the Li—Mg—O alloy, was confirmed on the Cufoil-side surface of the solid electrolyte layer of the evaluationbattery 4 after the first charge-discharge cycle.

The reason why the composite metal oxide contained in the protectivelayer contained the oxygen element, is as follows: once the metal layeris formed on the surface of the Cu foil by vacuum deposition, the metallayer reacts with oxygen in the air to form a metal oxide layer, and themetal oxide layer reacts with lithium ions during the evaluation batteryis charged.

TABLE 1 Average charge- Resistance Protective discharge Resistance (Ω)increase rate layer efficiency (%) 1st cycle 4th cycle Δ (%) Example 1Li—Sn—O 98.4 18.4 25.3 6.9 37.5 Example 2 Li—Al—O 98.5 20.3 22.6 2.311.3 Example 3 Li—Au—O 98.6 17.5 18.8 1.3 7.4 Example 4 Li—Mg—O 99.318.6 19.3 0.7 3.8 Comparative — 97.3 20.4 30.5 10.1 49.5 Example 1

Evaluation Result

For the evaluation battery 5 of Comparative Example 1 which did notcontain the protective layer, the average charge-discharge efficiencywas 97.3%, and the resistance increase rate after the 4thcharge-discharge cycle with respect to the first charge-discharge cycle,was 49.5% and high.

For the evaluation batteries 1 to 4 of Examples 1 to 4, each of whichcontained the protective layer, their average charge-dischargeefficiency was higher than the average charge-discharge efficiency ofthe evaluation battery 5 of Comparative Example 1, which did notcontained the protective layer, and their resistance increase rate waslower. Especially for the evaluation battery 4 of Example 4, which usedLi—Mg—O as the composite metal oxide, the average charge-dischargeefficiency was 99.3% and high; the resistance increase rate was 3.8% andlow; and the evaluation battery 4 showed excellent batterycharacteristics.

Accordingly, it was proved that the all-solid-state battery which isprovided with high charge-discharge efficiency and configured to largelysuppress an increase in resistance, is provided by the disclosedembodiments.

REFERENCE SIGNS LIST

-   11. Solid electrolyte layer-   12. Cathode layer-   13. Anode layer-   14. Cathode current collector-   15. Anode current collector-   16. Cathode-   17. Anode-   18. Protective layer-   100. All-solid-state battery

The invention claimed is:
 1. An all-solid-state battery, wherein theall-solid-state battery comprises a cathode comprising a cathode layer,an anode comprising an anode layer, and a solid electrolyte layerdisposed between the cathode layer and the anode layer; wherein theanode layer contains at least one selected from the group consisting ofa lithium metal and a lithium alloy; and wherein a protective layercomprising a composite metal oxide represented by Li—Mg—O is disposedbetween the anode layer and the solid electrolyte layer.
 2. Theall-solid-state battery according to claim 1, wherein a thickness of theprotective layer is from 30 nm to 300 nm.